There is a trend to do large portraits dedicated to the dark forces that play a role in fantasy literature and movies. The all-seeing eye of Sauron or evil dragons can be striking in their artwork and meaning.

There are certain animals that are also associated with evil that you could consider. Wolves, werewolves, vampires, bats, and snakes all suggest evil, and because you are exploring the fantasy space, you can make them as grotesque or unusual as you want.